Science // What causes a knee jerk response?

Hammer + Knee = Kick?

A knee jerk response. Remember when you were a kid (or even as an adult) and the doctor would tap a reflex hammer to your knee causing your foot to quickly kick? Well today I learned why this happens. I had no idea that this same movement is responsible for your head jerking up when you start to nod off during a boring presentation (I know I’m not the only one!) Kinesiology–the study of human movement–is fascinating!

Science behind these sudden reflexes

These sudden reflexes happen because your muscle spindles are sensitive to stretching and the rate they stretch. When a
stretch occurs rapidly, an impulse is sent to your central nervous system (CNS) that activates motor neurons of the muscle and causes them to contract. This knee jerk (also known as the patellar tendon reflex) happens when the reflex hammer strikes your
patellar tendon causing a quick stretch of the musculotendinous unit of your quadriceps. In other words, the hammer stretches your quadriceps muscle which reacts by contracting and kicking your foot forward–hence the name “knee jerk response”.

Another example of this is the jerking of your head when you start to drift off to sleep while sitting up (maybe in class or in my case when I try sleep on an airplane). When the neck extensors are stretched as your head nods forward, muscle spindles are
activated and you suddenly jerk your head up to correct your posture.

This rapid stretch can also be used to help you jump greater distances. When you do a short squat before jumping, this stretch results in more force being generated in your muscles to jump off the floor.